In recent years, glass products for use in the field of optical communication, semiconductor industry, and the like are required to have exceedingly high quality. The purities thereof are strictly regulated. Such high-purity glasses are mainly produced by the known method (1) in which a sandy natural quartz powder obtained by pulverizing natural quartz (this powder is generally called “sand”) is used as a raw material. Methods usable in the case where a glass having a higher purity is desired include: (2) the oxyhydrogen flame method which comprises decomposing silicon tetrachloride in an oxyhydrogen flame, depositing the resultant fume on a substrate to grow a fume deposit, and using this fume deposit as a raw material; and (3) a method in which a synthetic quartz powder obtained by the so-called sol-gel method from a gel formed from an organometallic compound such as, e.g., a metal alkoxide, is used as a raw material.
However, those methods each have had both merits and demerits. For example, in the method (1), since a natural quartz powder, which intrinsically contains metallic elements such as aluminum and iron in the quartz particles, is used as a raw material, it is difficult to obtain a quartz powder product highly purified to such a degree as to have a metal impurity content of 100 ppb or lower even when a purification operation, e.g., pickling, is repeated. In the method (2), production at a commercially acceptable low cost is difficult, although a high purity can be attained. This method hence has not come to be used for mass-production.
In the sol-gel method (3), on the other hand, there has been a drawback that the raw materials, intermediate, and product necessarily contact with the production apparatus and impurities come into these due to contacts with the apparatus, although the product can be mass-produced. In particular, the particles (sol or gel) and wet gel yielded by reactions of the organometallic compound as a raw material with an alkoxide and water come into contact with the inner wall of the apparatus and repeatedly undergo deposition, peeling, and shedding, during which abnormal particles (scaling debris) come into the product. Examples of the apparatus in which such scaling debris generate include a series of synthetic-quartz production apparatus and individual devices for these which each have a part coming into contact with at least the reaction liquid, wet gel, or dry gel. Specific examples thereof include the reactor, pulverizer, dryer, piping, and the like. It has been extremely difficult to separate and remove the scaling debris from the product.
When the gel is burned to produce a synthetic quartz powder, such scaling debris change into carbon ingredients. The carbon ingredients aggregate to form black contaminant particles in the product. It has further been known that when the synthetic quartz powder is melted in forming a glass molding, the carbon ingredients decompose into gases and these gases form bubbles in the glass molding and thereby significantly impair the quality of the glass molding.
An example of known methods for eliminating the problem described above is to regulate a synthetic quartz powder obtained by a sol-gel reaction so that the number of black particles present in the powder is reduced to 5 or smaller per 50 g (JP-A-8-188411). In this patent document, there also is a statement to the effect that the glass molding obtained from a melt of this synthetic quartz powder is more reduced in bubble inclusion than those obtained by related-art techniques.
However, the quality requirements which the glass products for use in the field of optical communication, semiconductor industry, and the like are required to satisfy are becoming severer. There has hence been a desire for the development of a synthetic quartz powder which is effective in inhibiting bubble inclusion to a level lower than that attained with such a related-art technique.
Another method is known for heightening the purity of a synthetic quartz powder produced by the sol-gel method. This method comprises sufficiently supplying air in a burning step in order to reduce the amount of the scaling debris which are thought to have come into the powder due to contacts with the production apparatus and which may come into the product and to prevent the scaling debris which have come into the powder from remaining as unburned carbon. It has been known that this technique is effective in obtaining a glass molding reduced in bubble inclusion. However, even this technique cannot satisfy recent requirements for a higher purity, and there has been a desire for a synthetic quartz powder effective in inhibiting bubble inclusion in a high degree. A method in which heat treatment is conducted under vacuum has also been proposed. However, this method has had a problem that industrial use thereof is difficult, for example, because to industrially realize vacuum conditions necessitates a high cost.